1. Field of the Invention
This invention relates to a fly-wheel permanent-magnet generator.
2. Description of the Related Art
In a conventional fly-wheel permanent-magnet generator, a bowl-shaped fly wheel having a magnet on the inner surface of its circumferential wall is used as a rotor. However, when the rotor rotates, an alternating field generated by the magnet causes hysteresis loss and eddy-current loss in a laminated core of a stator. The magnetic force and the quantity of imbalance of the rotor itself cause variance in the rotation locus. The rotation variance leads to variance in the rotation accuracy (quantity of air gap between the stator and the rotor) and largely affects the power generation characteristic (output characteristic).
Also, the fly-wheel permanent-magnet generator is often directly connected with a shaft of an internal combustion engine or the like, and the rotation variance from the shaft and the rotation variance of the generator itself adversely affect the shaft side. Therefore, in order to reduce the imbalance of the rotor (rotation imbalance), balancing (imbalance correction processing) is performed.
Moreover, in the fly-wheel permanent-magnet generator, generally, a vent that is as large as possible is installed to increase the ventilation efficiency. Since processing variance, variation of attached components (plural magnets are installed with a gap from each other) and the like cause a certain quantity of rotation imbalance, balance correction processing such as drilling needs to be performed many times in a later process.
To address this circumstance, in the conventional permanent-magnet generator having a fly wheel made of sheet metal, the size (area) of one or some of plural vents opened at equal spacing in the bottom wall part of the fly wheel is adjusted to maintain the rotation balance, for example, as disclosed in Patent Reference 1.
Patent Reference 1: JP-11-178299 (paragraphs 0031 and 0032, and FIG. 4)
Recently, a fly-wheel permanent-magnet generator tends to produce high output. Its fly wheel is produced by casting or hot forging, instead of using sheet metal, and is integrally formed with a central boss part of the bottom wall part. Meanwhile, multi-polarization of the core and magnet (magnetic pole) causes higher frequency, and the thickness of the generator is reduced in order to reduction in size and weight.
Therefore, it is necessary to minimize the quantity of rotation imbalance that affects the rotation rigidity.
However, if the fly wheel is produced by casting or hot forging, rotation imbalance is caused by variance in density (specific gravity) rather than dimensional variance. Therefore, it is difficult to reduce the quantity of rotation imbalance to the required level, simply by adjusting the size (area) of one or some of the vents as described above.
Moreover, even if the processing variance and the variation of attached components are addressed to improve the accuracy, it is difficult to completely eliminate the rotation imbalance, because of the influence of a key groove provided at the center of the boss part.